Amplitude modulation for quasi-phasematched nonlinear optical frequency converters

ABSTRACT

Quasi-phasematching design to provide an approximation to a desired spectral amplitude response A(f) is provided. An initial phase response φ(f) corresponding to A(f) is generated. Preferably, d 2 φ(f)/df 2  is proportional to A 2 (f). Alternatively, φ(f) can be a polynomial in f. A function h(x) is computed such that h(x) and H(f)=A(f)exp(iφ(f)) are a Fourier transform pair. A domain pattern function d(x) can be computed by binarizing h(x) (i.e., approximating h(x) with a constant-amplitude approximation). In some cases, the response provided by this d(x) is sufficiently close to A(f) that no further design work is necessary. In some embodiments of the invention, the need for binarization can be reduced or eliminated by providing amplitude modulation of the effective nonlinearity.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation in part of U.S. application Ser. No. 11/513,814, filed on Aug. 30, 2006, entitled “Design of Quasi-phasematched Optical Frequency Converters”. U.S. application Ser. No. 11/513,814 is a continuation in part of U.S. application Ser. No. 11/299,161, filed on Dec. 8, 2005, entitled “Design of Quasi-phasematched Optical Frequency Converters”.

FIELD OF THE INVENTION

This invention relates to nonlinear optical frequency conversion.

BACKGROUND

Nonlinear optical frequency conversion includes any process where one or more optical inputs provided to a nonlinear optical device produce one or more optical outputs, where the output radiation includes radiation at one or more frequencies (or wavelengths) not present in the input radiation. Examples of nonlinear frequency conversion include second harmonic generation (SHG), sum frequency generation (SFG), difference frequency generation (DFG), four wave mixing, third harmonic generation, parametric oscillation, etc. Many nonlinear optical processes require phase-matching to proceed efficiently. If the phase matching condition is satisfied, then the nonlinear interaction proceeds constructively along the entire active length of the device, while if the phase matching condition is not satisfied, then radiation from different parts of the nonlinear device interferes destructively to reduce conversion efficiency. As a result, investigation of such processes (e.g., second order processes such as DFG, SFG, and SHG) has concentrated primarily on methods for phase-matching.

The phase-matching condition can be expressed in geometrical terms. For example, phase-matching for SHG requires the wave vector of the second harmonic wave to be twice as long as the wave vector of the input wave (i.e., the pump wave). Due to material dispersion (i.e., the wavelength dependence of the index of refraction), the SHG phase matching condition is ordinarily not satisfied. Birefringent phase-matching (BPM) and quasi phase-matching (QPM) are two methods of phase-matching that have been extensively investigated. In BPM, birefringent materials are employed and the interaction geometry and wave polarization are selected such that the phase matching condition is satisfied. For example, the pump and second harmonic waves can have the same index of refraction to phase-match SHG. In QPM, the nonlinear device is spatially modulated to provide phase matching. For example, periodic spatial modulation of a nonlinear device can provide a device k-vector K such that 2k_(p)±K=k_(sh) to phase-match SHG, where k_(p) is the pump wave vector and k_(sh) is the second harmonic wave vector. A common method of providing spatial modulation for QPM is to controllably alter the sign of the nonlinear coefficient (e.g., by poling a ferroelectric material).

In more general terms, QPM can be regarded as a method for engineering the spectral response of a nonlinear optical device to provide various desirable results. From this point of view, periodic QPM is a special case of QPM that is especially appropriate for maximizing conversion efficiency at a single set of input and output wavelengths. Other design constraints can lead to various non-periodic QPM methods. For example, in U.S. Pat. Nos. 5,815,307 and 5,867,304 aperiodic QPM is employed in connection with frequency conversion of short optical pulses. Since short pulses include multiple wavelengths, periodic QPM optimized for a single wavelength is not preferred. In U.S. Pat. No. 6,016,214, a QPM grating having multiple sections, each having a different period, is employed to phase-match multiple nonlinear processes. In U.S. Pat. Nos. 6,714,569 and 5,640,405, QPM for two or more nonlinear processes simultaneously is also considered.

Increasing the wavelength acceptance bandwidth by periodic or aperiodic phase reversal is considered by Bortz et al., in Electronic Letters 30(1), pp 34-35, 1994. Sinusoidally chirped QPM is considered by Gao et al., in Photonics Technology Letters, 16(2), pp 557-559, 2004. Combination of a phase reversal grating and a periodic grating for QPM is considered by Chou et al., in Optics Letters, 24(16), pp 1157-1159, 1999.

In some cases, it is desirable to specify a nonlinear device spectral response (e.g., normalized SHG efficiency) over a range of frequencies. In such cases, the above-described methods may or may not be applicable, depending on whether or not the desired spectral response falls into the set of spectral responses provided by the method. For example, QPM gratings having several periodic sections provide a spectral response having several peaks, each peak having characteristic side lobes due to the sin(x)/x (i.e., sinc(x)) response from each grating section. If the desired spectral response is one or several sinc-like peaks, then this method is applicable. If the desired spectral response is different (e.g., the side lobes need to be eliminated), then this method may not be applicable.

Accordingly, it would be an advance in the art to provide QPM having a specified spectral response (or tuning curve).

SUMMARY

The invention enables QPM design to provide an approximation to a desired spectral amplitude response A(f). An initial phase response φ(f) corresponding to A(f) is generated. Preferably, d²φ(f)/df² is proportional to A² (f). Alternatively, φ(f) can be a polynomial in f. A function h(x) is computed such that h(x) and H(f)=A(f)exp(iφ(f)) are a Fourier transform pair. A domain pattern function d(x) can be computed by binarizing h(x) (i.e., approximating h(x) with a constant-amplitude approximation). In some cases, the response provided by this d(x) is sufficiently close to A(f) that no further design work is necessary. In other cases, the design can be iteratively improved by modifying φ(f) responsive to a difference between the desired response A(f) and the response provided by domain pattern d(x). Various approaches for binarization are provided. The availability of multiple binarization approaches is helpful for making design trades (e.g., in one example, fidelity to A(f) can be decreased to increase efficiency and to increase domain size). In some embodiments of the invention, the need for binarization can be reduced or eliminated by providing amplitude modulation of the effective nonlinearity.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a QPM design method according to an embodiment of the invention.

FIG. 2 shows an example of a spectral conversion efficiency specification.

FIGS. 3 a-c show a first example of QPM design according to an embodiment of the invention.

FIGS. 4 a-c show a second example of QPM design according to an embodiment of the invention.

FIG. 5 shows an example of QPM design according to another embodiment of the invention.

FIG. 6 shows a QPM design method according to another embodiment of the invention.

FIGS. 7 a-e show various ways of providing amplitude modulation of the effective nonlinearity in a waveguide nonlinear frequency converter.

DETAILED DESCRIPTION

FIG. 1 shows a QPM method according to an embodiment of the invention. Step 102 on FIG. 1 is specifying a target spectral amplitude response A(f). The response A(f) describes the desired spectral (i.e., as a function of wavelength) performance of the QPM device being designed. More specifically, A²(f) is the desired relative spectral power efficiency of the device, so A(f) is referred to as an amplitude response. In practice, A²(f) may be given, from which A(f) can readily be derived. Thus the QPM design problem can be regarded as choosing a domain pattern function d(x) that provides a spectral amplitude response B(f) that is sufficiently close to the target spectral amplitude response A(f). The specification of A(f) should span the frequency range(s) of interest, including any range(s) where zero response is desired.

Here it is assumed that QPM is accomplished by changing the sign of the nonlinear coefficient in a controlled manner (e.g., by controlled domain poling of a ferroelectric nonlinear material). Under this assumption, the domain pattern function d(x) has sign reversals at each domain boundary, but |d(x)| is a constant. It is convenient (but not required) to normalize the calculations such that |d(x)|=1. With this normalization of d(x), it is clear that the amplitude response A(f) can be correspondingly normalized. Thus the response A(f) is to be regarded as a relative response, in the sense that multiplication of A(f) by a wavelength-independent factor does not essentially change the QPM design problem being specified. Such normalization is well known in the art, as is the equivalence between normalized and non-normalized QPM formalisms.

It is also well known in the art that there is a Fourier transform relation between the domain pattern function d(x) and spectral response D(f) of a QPM nonlinear device. More specifically, d(x) and D(f) are a Fourier transform pair. Here (and throughout this description) x is position and f is frequency or any suitable equivalent such as spatial frequency or angular frequency. For example, a wavelength of 1 μm corresponds to a spatial frequency of 1 μm⁻¹ and to a frequency of 300 THz. Since various conventions exist for defining Fourier transforms, no significance attaches to convention-dependent details such as the location of factors of 2π, sign conventions, the use of one-sided or two-sided transforms, and whether D(f) is regarded as the Fourier transform or inverse Fourier transform of d(x). Any mutually consistent Fourier transform formalism will suffice for practicing the invention.

Practice of the invention also does not depend on whether these Fourier transform pairs are continuous or discrete. In most cases, including the following examples, discrete Fourier transforms are preferred to make use of efficient algorithms such as the fast Fourier transform.

Even though there is a simple relation between d(x) and D(f), it does not follow that d(x) can readily be derived from the desired response A(f). To appreciate this, it is helpful to define the amplitude (B(f)) and phase (ψ(f)) responses of a QPM device via D(f)=B(f)exp(iψ(f)), where B(f) is real and non-negative and ψ(f) is real. Now it is clear that specifying A(f) is only a partial specification of the desired spectral response, since the phase is left unspecified. We assume that the designer does not care directly about the phase response (e.g., the common case where efficiency B²(f) is the quantity of concern).

However, it does not follow that the phase response is irrelevant to QPM design. In fact, a good choice of phase response can improve QPM design (i.e., make B(f) closer to A(f)) and a poor choice of phase response can degrade QPM design (i.e., make B(f) farther from A(f)), other things being equal. The reason for this is that d(x) is constrained to have a constant amplitude, and varying the phase response affects the influence this constraint has on design fidelity (i.e., how closely B(f) can approach A(f)).

Accordingly, step 104 on FIG. 1 is providing an initial phase response φ(f). The phase response φ(f) can be regarded as a target phase response specification selected to provide good fidelity between A(f) and B(f). Preferably, φ(f) is selected such that d²φ(f)/df² is proportional to A2(f) and more preferably, ϕ(f) ∝ ∫₀^(f)(∫₀^(z₂)A²(z₁)𝕕z₁)𝕕z₂. This choice of phase response works well in practice, and often provides a non-iterative design solution. The proportionality constant is preferably set to minimize the standard deviation of the values of h²(x) divided by the average value of h² (x). Thus, the proportionality constant is set to make h(x) as uniform as possible given other constraints. Standard numerical methods are applicable for determining this constant of proportionality, and since this is a one-dimensional optimization problem, it is not computationally intensive. Other initial target phase responses can also be employed, e.g., in connection with iterative design methods as described below. Specification of the phase response as described herein is a key aspect of the invention.

In addition to having a constant amplitude, d(x) is a real-valued function. Therefore, the responses A(f) and φ(f) are required to have the corresponding Fourier symmetries (i.e., A(f)=A(−f) and φ(f)=−φ(−f)). Step 106 on FIG. 1 is calculating h(x) such that h(x) and H(f)=A(f)exp(iφ(f)) are a Fourier transform pair. The symmetry conditions on A(f) and φ(f) ensure that h(x) is a real-valued function. If the domain pattern function d(x) could be set equal to (or proportional to) h(x), the QPM design problem would be solved. However, h(x) generally does not have a constant amplitude, so the domain pattern function d(x) needs to be a constant-amplitude approximation to h(x).

It is helpful to regard the process of deriving a constant-amplitude approximation to h(x) as “binarizing” h(x). Accordingly, step 108 on FIG. 1 is binarizing h(x) to provide d(x). Such binarization can be accomplished in various ways.

One method of binarization is to set d(x) proportional to sgn(h(x)), which effectively looks at only the sign of h(x). Other binarization methods that have been developed can be grouped into two categories. In the first category, d(x) is set proportional to sgn(h(x)−m(x)), where m(x) is a modulating waveform. In the second category, d(x) is set proportional to sgn(g(x)−m(x)), where m(x) is a modulating waveform and g(x) is an average of h(x). In addition to these two categories, it is also possible to include frequency domain processing in the binarization process.

In the first category, d(x) is set proportional to sgn(h(x)−m(x)), where m(x) is a modulating waveform preferably selected to provide a D(f) that is substantially proportional to H(f) over a predetermined range of f. Selection of the range of f is problem-dependent and within the skill of an average art worker. Achieving this condition provides good fidelity between A(f) and B(f). Suitable modulating waveforms include triangle waves and sawtooth waves. A triangle wave has equal positive and negative slopes in each period and has no discontinuities, while a sawtooth wave has only one slope in each period and has discontinuities. Triangle wave modulation is preferred to sawtooth wave modulation because the resulting domain sizes tend to be slightly larger. In cases where m(x) is periodic with period T, it is preferred for 1/T to be greater than about three times the largest frequency of interest specified by A(f). The amplitude of m(x) is usually set equal to the peak amplitude of h(x), although other choices are possible for the amplitude. Random or pseudorandom waveforms for m(x) are also suitable for practicing the invention. In this description, pseudo-random refers to a deterministic output (e.g., from a random number generator) that passes statistical tests for randomness. Spectral filtering of a random or pseudo-random input can be employed to provide a filtered random or pseudo-random m(x). In practice, good results have been obtained with a pseudo-random m(x) having an amplitude uniformly distributed within a predetermined range.

The second category is like the first, except that h(x) is averaged to provide g(x) inside the sgn() function. This average can be a moving average. Alternatively, in cases where m(x) is periodic with period T, g(x) can be given by ${g(x)} = {\frac{1}{T}{\int_{{\lfloor{x/T}\rfloor}T}^{{({{\lfloor{x/T}\rfloor} + 1})}T}{{h(y)}{{\mathbb{d}y}.}}}}$ Here └z┘ (i.e., floor(z)) is the largest integer ≦z. Thus within each period i of m(x), an average value of h_(i) of h(x) is computed. The value of g(x) is the average hi corresponding to the period containing x. Averaging h(x) to g(x) desirably tends to increase domain size, but in turn tightens tolerances on duty cycle control. To maximize domain size in cases where m(x) is periodic with period T and averaging of h(x) to g(x) is performed, it is preferred for 1/T to be greater than about two times the largest frequency of interest specified by A(f). The selection of what kind of averaging to employ (if any) can be made by an art worker based on overall design and process considerations, taking into account the minimum domain size limit and duty cycle tolerances of the available domain patterning processes.

Binarization can also include frequency-domain processing. More specifically, the D(f) corresponding to a domain pattern function d(x) can be processed to provide a modified function F(f). A function f′(x) is computed such that f′(x) and F(f) are a Fourier transform pair, and f′(x) is binarized to provide f(x) (with any of the above binarization methods). Such processing can provide various useful enhancements to binarization. For example, low pass filtering of D(f) to provide F(f) increases the domain size in f(x) compared to d(x). Increasing the domain size (especially the minimum domain size) is helpful for fabrication, since smaller domains tend to be more difficult to fabricate than larger domains. The response D(f) can also be filtered to reject QPM harmonics. Spectral processing in combination with binarization can be an iterative process or a single-pass process. In an iterative process, the processing of D(f) to obtain F(f), the calculation of f′(x), and the binarization of f′(x) to obtain f(x) are repeated in sequence until a termination condition is satisfied.

In performing iterative binarization, it has been found helpful to monitor the convergence of the process. If a condition of slow convergence is detected, the iteration can be restarted with different initial conditions. Iteration after such restarting is found to converge quickly in most cases, making such restarting a surprisingly effective strategy. In cases where m(x) is random or pseudorandom, the restarting is performed such that a different m(x) is generated. In other cases (e.g., if m(x) is a triangle wave or sawtooth wave) the phase and/or amplitude of m(x) can be perturbed (deterministically or randomly) as part of the restarting, to avoid encountering the same convergence difficulty. Here the phase of m(x) refers to the relative phase between m(x) and h(x).

FIG. 1 also shows optional steps 110, 112, and 114. Optional step 110 is computation of the response D(f)=B(f)exp(iψ(f)) provided by the domain pattern function d(x). Since the goal is to make B(f) as close as possible to A(f), standard numerical methods are applicable. A figure of merit depending on A(f) and B(f) can be selected (e.g., an integral of the square of the difference between A(f) and B(f) in a predetermined range or ranges of f)). The input phase response φ(f) (typically in a discretized approximation) can be varied to improve the figure of merit (step 112). Such variation can be iterated (step 114) according to known numerical optimization techniques.

FIG. 2 shows an example of a spectral conversion efficiency specification. The target response of FIG. 2 is used in the examples of FIGS. 3 a-c and 4 a-c. FIG. 2 shows normalized conversion efficiency, so the corresponding target amplitude response A(f) is the square root of the plot of FIG. 2.

FIGS. 3 a-c show a first example of QPM design according to an embodiment of the invention. FIG. 3 a shows the spectral response (i.e., B²(f)) plotted between 0.5 dB limits computed from the target spectral response of FIG. 2. Excellent response fidelity is observed in this example. FIG. 3 b shows a section of the domain pattern function d(x) in a 50 μm window 1 mm into the device, and FIG. 3 c shows the spectral response over a larger range of spatial frequencies. Binarization-induced peaks (near 0.5 μm⁻¹, 1.7 μm⁻¹ and 2.8 μm⁻¹) are visible, as are images of the tuning curve. In this example, the phase response is proportional to ∫₀^(f)(∫₀^(z₂)A²(z₁)𝕕z₁)𝕕z₂, the proportionality constant is optimized as described above, and sgn(h(x)−m(x)) binarization is employed where m(x) is a triangle wave having a fundamental frequency of 0.57 μm⁻¹ (3 times the largest specified frequency in A(f)). The amplitude of m(x) is set equal to the peak amplitude of h(x).

FIGS. 4 a-c show a second example of QPM design according to an embodiment of the invention. In this example, a trade is made to increase efficiency and domain size at the expense of somewhat reduced response fidelity. This example is the same as the example of FIGS. 3 a-c, except that the amplitude of m(x) is decreased by a factor of 3π/4. FIG. 4 a shows the spectral response (i.e., B²(f)). FIG. 4 b shows a section of the domain pattern function d(x) in a 50 μm window 1 mm into the device, and FIG. 4 c shows the spectral response over a larger range of spatial frequencies. Comparison of FIGS. 4 b and 3 b show far fewer short domains in FIG. 4 b, which can greatly facilitate fabrication. Correspondingly, the high spatial frequency content on FIG. 4 c is much less than on FIG. 3 c. Considerable design flexibility is provided by the various binarization approaches, and selection of suitable binarization approaches for particular designs in accordance with principles of the invention is within the skill of an art worker.

In another embodiment of the invention, the phase response φ(f) is represented as a polynomial. Polynomial coefficients can be employed as fitting parameters in an iterative QPM design process. More specifically, ${\phi(f)} = {\sum\limits_{i = 0}^{n}{A_{i}{p_{i}(f)}}}$ can be assumed for the initial phase response, where p_(i)(f) are basis polynomials having degree ≦i, A_(i) are the polynomial coefficients, and n is the number of terms in the polynomial expansion. Any convenient set of polynomials can be employed as the basis polynomials. Sets of polynomials that have been investigated for use as such a basis include powers (e.g., 1, x, x², x³, . . . ), Legendre polynomials and Chebyshev polynomials. It is preferred to select a basis of orthonormal polynomials, since orthonormal polynomials tend to provide improved numerical convergence in practice. Chebyshev polynomials are one type of orthonormal polynomials that have provided good results in work to date. Evaluation of various sets of polynomials for use as a basis in this method is within the skill of an art worker.

The use of a polynomial initial phase function is helpful for suppressing oscillations in the QPM device tuning curve that can occur as design constraints are imposed. Suppression of such oscillations can be particularly helpful for narrow linewidth sources and/or in cases where the frequency converter is employed inside a resonant device. Two methods of fitting the polynomial coefficients to provide QPM designs have been investigated.

In the first method, the coefficients A_(i) are determined by a numerical fitting procedure designed to reduce the impact of binarization as described above. More specifically, the coefficients A_(i) can be determined by numerical minimization of h²(x)−H₀, where H0 is a fitting parameter. Since binarization of h(x) would introduce no difference between the actual tuning curve and the desired tuning curve if |h(x)| were constant, it is helpful to select an initial phase function that leads to an h(x) that most closely approaches this ideal. The preceding optimization minimizes the mean-square deviation of h(x) from a constant amplitude function. In practice, it is preferable (but not required) to initiate the optimization with a relatively small set of basis polynomials (e.g., 2 to 4). Successive refinements are then made to the optimization by adding 1-2 additional basis polynomials to the set and re-optimizing, using the results of the previous iteration as a starting point. In this manner, difficulties associated with finding a suitable starting point for a high-dimensional optimization can be avoided. This process can be continued until no further improvement in fit is obtained, until the fit reaches a predetermined level of accuracy, and/or until computational artifacts occur. As many as 50 coefficients have been employed in practice, when a power basis was employed. Orthonormal polynomials tend to provide reduced order, with no more than about 30 coefficients being employed in practice.

Once the phase coefficients have been determined in this manner, the resulting optimized h(x) can be binarized as described above. In cases where orthonormal polynomials are employed, high chip rates can occur at the ends of the domain pattern, which can undesirably result in small domains. Binarization methods as described above can be employed to mitigate this small domain issue.

FIG. 5 shows results from a design according to this embodiment of the invention. Here the solid line is the desired tuning curve, and the dashed line is the tuning curve provided by a QPM design according to the invention. In this example, Chebyshev polynomials were employed to provide the most rapid convergence and best fit for n=20, and where integrated square deviation was employed as a fitting measure. A good fit with <1% RMS deviation was obtained.

A characteristic feature of devices designed according to the above-described methods relating to a polynomial phase function is the presence of high order terms in φ(f) (i.e., terms in a polynomial fit or power expansion of φ(f) that are 4th order and higher), in a device having a non-sinusoidal φ(f). To appreciate this aspect of the invention, it is helpful to note that prior QPM design approaches can provide a φ(f) having either of two functional forms: 1) φ(f) is substantially sinusoidal, or 2) φ(f) is substantially a polynomial in f having degree ≦3. Thus the methods of the invention provide a systematic framework within which to optimize a high order, non-sinusoidal φ(f) to improve design fidelity to the desired tuning curve.

Accordingly, devices having a high order, non-sinusoidal φ(f) are also provided by the invention. More specifically, devices of the invention have a domain pattern d(x) and a complex frequency response D(f) that are a Fourier transform pair. The complex frequency response D(f)=A(f) exp (iφ(f)), where A(f) is a non-negative real amplitude response and φ(f) is a real phase response. The phase response φ(f) is substantially non-sinusoidal and includes substantial contributions from 4th and higher order terms in a polynomial fit.

Operationally, whether or not φ(f) is substantially sinusoidal can be assessed by computing a sinusoidal best fit ψ(f) to φ(f). A test complex frequency response D′(f)=A(f) exp (iψ(f)) and a test domain pattern d′(x) can be computed such that d′(x) and D′(f) are a Fourier transform pair. If d′(x) differs substantially from d(x) (e.g., by having >10% imaginary components, or otherwise), then φ(f) is substantially non-sinusoidal.

Similarly, whether or not φ(f) includes substantial 4th and higher order contributions can be assessed by computing a best fit ψ(f) to φ(f), where ψ(f) is a third order fitting polynomial (i.e., ψ(f)=A₀+A₁f+A₂f²+A₃f³). A test complex frequency response D′(f)=A(f) exp (iψ(f)) and a test domain pattern d′(x) can be computed such that d′(x) and D′(f) are a Fourier transform pair. If d′(x) differs substantially from d(x) (e.g., by having >10% imaginary components, or otherwise), then φ(f) includes substantial 4th and higher order contributions.

In the preceding description, binarization is employed to impose the constraint that the effective nonlinearity have constant amplitude, as is often the case in poled ferroelectric devices. However, the invention can also be practiced in combination with various techniques for providing amplitude modulation of the effective nonlinearity, thereby reducing or eliminating the need for binarization.

FIG. 6 shows a method according to such an embodiment of the invention. The method of FIG. 6 is like the method of FIG. 1, except that amplitude modulation 602 (on FIG. 6) is employed instead of binarization 108 (on FIG. 1). More specifically, step 602 details determining a real constant amplitude domain function d(x) and a real amplitude modulation function q(x) such that the product d(x)q(x) is an approximation to h(x). Amplitude modulation of the effective nonlinearity can provide various advantages. For example, the degrees of freedom thereby introduced can be employed to increase the minimum domain size and/or to improve fidelity between the desired response and the actual response and/or to reduce design iteration time.

The division of h(x) into the factors d(x) and q(x) can be performed in various ways. Typically it is preferable to select d(x) and q(x) such that both factors are relatively easy to realize in practice. For example, a minimum domain length constraint can be imposed on d(x). As another example, d(x) can be selected such that the resulting q(x) has a relatively small range of variation. Initial phase functions as described above for reducing the errors induced by binarization should also provide good results for amplitude modulation embodiments, since reducing binarization error is analogous to reducing the range of variation of q(x).

FIGS. 7 a-d show some examples of how such amplitude modulation can be realized for waveguide devices. In these examples, active region 702 is a region including a nonlinear optical material that can be poled according to a domain pattern. Various domain patterns can be employed. For example, in FIG. 7 a, domain pattern 704 is laterally patterned such that some domains completely cross waveguide 706, while other domains do not completely cross waveguide 706. Domains that have an incomplete overlap with the waveguide mode make a reduced contribution to the effective nonlinearity. Such lateral patterning of the domain width within the active region provides amplitude modulation of the effective nonlinearity. The example of FIG. 7 a shows a configuration suitable for apodization of the frequency response of the nonlinear optical frequency converter, since the effective nonlinearity smoothly decreases at both ends of the device. Such apodization is often desirable in practice, since it is helpful for suppressing undesirable nonlinear interactions outside the frequency band of interest.

Such lateral patterning of the domain width can also provide arbitrary amplitude modulation, as shown in the example of FIG. 7 b. Here domain pattern 708 and waveguide 706 combine to provide apodization at the device ends combined with modulation along the length of the device.

In the preceding examples, amplitude modulation is provided by lateral patterning of the domain width. It is also possible to provide amplitude modulation in devices having a fixed domain width within the active region, as shown in FIG. 7 c-d. The example of FIG. 7 c shows a waveguide 712 having tapered input and output passing through a constant-width domain pattern 710. Even though the domains all have constant width, the overlap between the waveguide mode and the domain pattern (which defines the effective nonlinearity) varies along the length of the device in this example. The reason for this is that the overlap depends on the size of the waveguide mode, which in turn depends on the size of the waveguide. As the mode size increases, the overlap of the mode with the domain pattern decreases. Thus the example of FIG. 7 c also provides apodization of the nonlinear response.

Amplitude modulation can also be provided by altering the transverse position of the waveguide with respect to a transverse boundary of active region 702. FIG. 7 d shows an example of such an embodiment. In this example, waveguide 714 follows a “wandering” path with respect to the lateral edge of active region 702. Parts of the waveguide having a large overlap with domain pattern 710 make relatively large contributions to the nonlinear response, while parts of the waveguide having a small overlap with domain pattern 710 make relatively small contributions to the nonlinear response. Amplitude modulation is thereby provided. Any combination of these methods can also be employed for providing amplitude modulation. For example, FIG. 7 e shows a waveguide 712 having tapered input and output in combination with altering the width of domain pattern 716 in the waveguide taper regions. This arrangement can provide increased amplitude modulation compared to waveguide tapering alone or varying the domain pattern width alone.

In some cases, binarization and amplitude modulation can be employed together, e.g., to improve dynamic range. For example, in an optimization of the initial phase response to maximize the flatness of h²(x), it can be advantageous to introduce a modulation function q(x) to ease numerical design. As another example, any of the above binarization procedures can be employed to provide a constant amplitude d(x) as an approximation to h(x). The modulation function q(x) can then be set equal to h(x)/d(x). Such modulation functions will advantageously tend to have a relatively small range of variation. Undesirable tuning curve ripple can be suppressed by tapering h(x) at the ends of the device (i.e., apodization), and/or by altering h(x) at other appropriate points along the length of the device. Design methods according to the present invention can be employed to determine how h(x) should vary along the entire length of the device.

Amplitude modulation is described above in connection with design methods according to the invention. Other embodiments of the invention relate more specifically to structures for providing amplitude modulation. In particular, some preferred embodiments of the invention are waveguide nonlinear frequency converters having amplitude modulation of the effective nonlinearity in constant domain width structures as in FIGS. 7 c-d. 

1. A method for designing a quasi-phasematched optical frequency converter, the method comprising: a) specifying a non-negative real target spectral amplitude response A(f), wherein A(−f)=A(f); b) providing a real initial phase response φ(f) associated with A(f), wherein φ(f)=−φ(−f); c) calculating a real function h(x) such that h(x) and H(f)=A(f) exp(iφ(f)) are a Fourier transform pair; d) determining a real constant-amplitude domain pattern function d(x) and a real amplitude modulation function q(x) such that d(x)q(x) is an approximation to h(x).
 2. The method of claim 1, wherein said providing a phase response comprises providing φ(f) such that d²φ(f)/df² is proportional to A²(f).
 3. The method of claim 1, wherein h(x) and H(f) are a discrete Fourier transform pair.
 4. The method of claim 1, wherein h(x) and H(f) are a continuous Fourier transform pair.
 5. The method of claim 1, wherein φ(f) is a polynomial in f.
 6. The method of claim 5, wherein said providing a real initial phase response comprises determining coefficients of said polynomial to minimize a deviation of h²(x) from a constant value.
 7. The method of claim 6, wherein said coefficients of said polynomial are coefficients of members of a predetermined polynomial basis set.
 8. The method of claim 7, wherein said predetermined polynomial basis is selected from the group consisting of a power basis, a Legendre polynomial basis, a Chebyshev polynomial basis, and a basis of orthonormal polynomials.
 9. A quasi-phasematched optical frequency converter comprising: a quasi-phasematched active region having a real constant-amplitude domain pattern function d(x) and having a real amplitude modulation function q(x); wherein an effective pattern function d′(x)=d(x)q(x) has a non-constant amplitude; wherein a function D(f) and d′(x) are a Fourier transform pair; wherein a non-negative real amplitude response A(f) and a real phase response φ(f) of the active region are defined via D(f)=A(f) exp(iφ(f)); wherein d²φ(f)/df² is substantially proportional to A²(f).
 10. A quasi-phasematched optical frequency converter comprising: a quasi-phasematched active region having a real constant-amplitude domain pattern function d(x) and having a real amplitude modulation function q(x); wherein an effective pattern function d′(x)=d(x)q(x) has a non-constant amplitude; wherein a function D(f) and d′(x) are a Fourier transform pair; wherein a non-negative real amplitude response A(f) and a real phase response φ(f) of the active region are defined via D(f)=A(f) exp(iφ(f)); wherein φ(f) is substantially non-sinusoidal; and wherein φ(f) includes a substantial contribution from 4th and higher order terms in a polynomial fit.
 11. A quasi-phasematched optical frequency converter comprising: a quasi-phasematched active region having a real constant-amplitude domain pattern function d(x); an optical waveguide passing through part or all of said active region; wherein an overlap of an optical mode of said waveguide with said active region is varied along a length of said active region to provide an effective domain pattern function d′(x) having non-constant amplitude; wherein said overlap is varied by altering a transverse waveguide profile along said length of said active region; wherein said active region has a constant transverse domain width along said length.
 12. The optical frequency converter of claim 11, wherein said overlap is varied by varying a transverse position of said waveguide relative to a transverse boundary of said active region.
 13. The optical frequency converter of claim 11, wherein said overlap is varied by tapering said waveguide.
 14. The optical frequency converter of claim 11, wherein said frequency converter has an input and an output, wherein said overlap is varied to smoothly decrease as said output is approached from within said converter, and wherein said overlap is varied to smoothly decrease as said input is approached from within said converter, whereby apodization of said converter is provided. 